Sup35p is a yeast protein with an essential function in promoting translation termination. [PSI] is believed to be a prion form of Sup35p. I have isolated several mutations that affect [PSI] behavior as a first step in identifying and characterizing genetic factors involved in various aspects of prion metabolism. [PSI] is eliminated from yeast cells during growth in the presence of millimolar levels of guanidine-hydrochloride (GuHCl) by an unknown mechanism. I have isolated mutant strains that are capable of maintaining [PSI] despite the presence of GuHCl in the growth medium. Six of these have been found to have a lesion at a single genetic locus and for one, complementing plasmids from a yeast genomic library have been isolated and are currently being characterized. The remaining mutants are undergoing preliminary genetic analysis and their affected genes are being isolated. [PSI] is extremely stable during mitosis and is invariably transmitted to all meiotic progeny. I have isolated mutant strains in which [PSI] has become unstable and is lost at elevated rates during mitotic growth. These mutants are currently being evaluated genetically. I have also discovered that certain combinations of known mutations lead to instability of [PSI] during meiosis. This meiotic instability is being analyzed to determine if the defect is manifested during the process of meiosis or if it resides in the inability of the meiotic progeny to support [PSI]. I have also discovered that a known mutation has significant effects on termination of translation independent of effects on [PSI]. Such a role for this gene has yet to be described. The genetic and molecular characterization of these effects is being pursued.